The present invention relates to a filter medium for use in raw water filter.
Generally, filtering of raw water is determined in light of the nature of the raw water, amount of organic material contained in the raw water, processing amount thereof, purity of the raw water, processing amount per unit time, etc. These factors are dependent upon filtering resistivity of a filter medium for use in a filter device, so that the characteristic of the filter medium is one of the important parameters for the filtering operation. Further, in case the filter medium is formed of a layer of particles, filtering resistivity is generally represented by the following equation: ##EQU1## in which, ho: filtering resistivity (water head m)
K: constant PA1 u: filtering velocity (m/S) PA1 L: thickness of filter medium bed (m) PA1 .mu.: viscosity of water (kg/m.multidot.S) PA1 .rho.: density of water (kg/m.sup.3) PA1 d: particulate size (m) of filter medium PA1 g: acceleration of gravity (m/S.sup.2) PA1 .epsilon.: porosity of filter medium bed
Incidentally, the equation (1) concerns the filter medium formed of a particulate bed. However, a mutual relationship also exists between each of the factors in equation (1) in case the filter medium is formed of filtering fabric, in which case partial modification of the equation is necessary.
In the equation, the function of porosity (1-.epsilon.).sup.2 /.epsilon..sup.3 exhibits the tendency of filtering resistance. The relationship between the porosity and the function of porosity is by a graph shown in FIG. 11. According to the graph, it is understood that if the porosity .epsilon. is decreased, the function of porosity (1-.epsilon.).sup.2 /.epsilon..sup.3 is abruptly increased, so that the filtering resistivity ho is also rapidly increased. As a result, the filter medium must be cleaned. That is, if the porosity .epsilon. is lower than a predetermined level, the filtering resistivity is increased exponentially, and as a result, the filter medium does not provide filterability. Further, in any kind of filter medium, the porosity .epsilon. is necessarily in the range of 0&lt;.epsilon.&lt;1. Therefore, the porosity is proximate to 1 in order to reduce filtering resistance. However, as the porosity increases to 1, materials such as organic materials to be trapped by the filter medium pass therethrough, to degrade the filtering function of the filtering device. Therefore, according to the conventional filtering device, porosity of the filter medium is lowered, thereby decreasing the processing amount per single circulation, and repeated filtering and cleaning of the filter medium is necessary so as to solve these conventional drawbacks.
Moreover, in order to filter water which exists in nature, the, nature of the water is closely related to the filtering condition. Contamination of water may be distinguished by the following four grades:
(a) where there is low content of dissolved oxygen, organic materials and muds are mostly oxidized and constituents are of inorganic material, and particularly, there is a small amount of colon bacilli, and further, there are small plankton and algae, and furthermore there are green algae, red algae, larvae of insects, etc.
(b) where there is high content of dissolved oxygen, oxidation of organic materials occurs, and an ammonia compound exists which is a product of the oxidation decomposition of the organic materials, and therefore, there are many kinds of a living thing such as diatom, green algae, conjugation algae, colon bacillus, Suctoria, bright moss, and fish.
(c) where organic material is decomposed, and black mud disappears, there are a large number of bacteria and algae, and further colon bacillus and Suctoria alive so that fish can inhabit in the water.
(d) where there exists a large content of organic material and oxidation decomposition occurs continuously, therefore mud color is black and further fish and algae can not inhabit due to a large amount of decomposed products of organic material.
As mentioned above, state of water is changeable due to amount of incoming organic material, water quality, and water condition. In case of filtering of water, if the water contains dissolved oxygen to allow existence of aerobic bacteria therein, algae and organic materials trapped by the filter are decomposed by the aerobic bacteria, whereas if water contains less dissolved oxygen, there is an anaerobic bacterial effect on the trapped materials. In both cases, continuous operation for a long period of time is required to conduct the decomposition on the filter medium by these bacteria.
During filtering operation, voids of the filter medium are closed within relatively short period of time, to require cleaning of the filter. Cleaning per se is troublesome and takes a long time.
In view of the above, according to the conventional filtering, the filter medium has low porosity in order to enhance trapping effect of the organic materials. Therefore, if the filtering resistance is increased due to adhesion of the organic material to the surface of the filter medium, the filter medium was subjected to cleaning at relatively short time intervals. Therefore, such filter medium is not capable of continuous filtering for a long period of time.